Whether Photons Do In Fact Age

[SolomonSnake]

Hi I was posting on io9 on a thread that asked "what scientific fact transformed the way you view the world?"

I posted

When I learned that photons, particles of light, do not age. That they are the same age they were at the beginning of the Big Bang, to illuminating the world around me, and until the death of the last star. I'd always known that when you travel closer to the speed of light time slows down, but I never quite made the leap to understanding what that means for light itself and that it is arrested in time.

I got that from Brian Greene in The Elegant Universe.

...in the majority of circumstances (slow speeds) most of an objects motion is through time, not space...the maximum speed through space occurs if all of an objects motion through time is diverted to motion through space...thus light does not get old; a photon that emerged from the big bang is the same age today as it was then. There is no passage of time at light speed.

Someone responded challenging me with.

It's not, really; in Special Relativity light always follows a null geodesic, and if you rearrange your system of coordinates so that it holds still, everything else follows a null geodesic. You can still play games with the line element in General Relativity so that when you hold a particle of light still, everything that is not light continues to follow spacelike geodesics. This, however, requires curved spacetime (which is not a feature of Special Relativity, which is built on the flat spacetime (Minkowski space) metric).

A more technical explanation would get pretty wordy since SR and GR have vastly different concepts of mass and energy-momentum itself is self-gravitating in the latter. Very very roughly, when you hold a particle of light still in a frame of reference, its momentum is transferred to the gravitational field, which in turn affects the notional "clocks" that ride along with other objects, such that the centre-of-momentum-frame photon sees them ticking faster in a higher gravitational potential (essentially like orbiting atomic clocks have a gravitational blueshift to us on the ground — ignoring the velocity differences, GPS satellites' time codes are a little faster than they would be in a lab on the ground, and in turn they see sea-level atomic clocks, at a lower gravitational potential (as they have less far to fall to the middle of the Earth) ticking a little slower than they would be in orbit).

So your held-still photon still gets a view of the universe around it as ticking along, and so the held-still photon can in principle calculate the passage of time in a universe which has both slower-than-light objects in it and Einstienian gravity.

I'm really having a hard time making sense of his reasoning and why gravity changes this. Also the term "held-still photon" isn't something I've ever come across. Maybe I shouldn't have used the phrase "arrested in time" or something. I'm not looking to one up this guy. I'd just really like this to be clarified.

I've spent at least an hour looking into this and I'm pretty sure I'm not mistaken, but I really can't decipher this guys comment.

 

[Orodruin]

You cannot decipher it because it does not make much sense. Light follows null geodesics regardless of the coordinate system chosen and massive objects follow time-like paths. This is an inherent property of how objects behave in space-time and has nothing to do with changing coordinates.

Sure, you can find a coordinate system where a light signal follows a coordinate line, but this coordinate will then have a zero in the diagonal component of the metric tensor for this coordinate.